In order to provide true mobility for a device connected to a wireless communication network across a large area, wireless data access for the area must be provided by a plurality of access points, or AP. In order to be able to provide data communication, a mobile terminal must first join the network. In this process, which is commonly termed association, the mobile terminal, or MT, associates with one of the APs in its neighborhood. At any given instant a particular AP, i.e. the one with which the MT is associated, acts as the serving AP for the MT. A MT is typically associated to a single AP at any time. Once the connection between the MT and the AP that the MT is currently associated to is beginning to fail to provide proper communication service, e.g. when the MT moves out of a service area in which the AP provides a good signal, it may be about time to disassociate the MT from the current AP and to connect and associate the MT to a different AP providing a better signal, e.g. a stronger signal or a signal suffering less from disturbance or interference, and thus providing proper communication service.
Typically an environment with multiple APs has each of these APs operating on a different, non-overlapping channel in order to avoid that the APs are creating hidden nodes among each other.
Many wireless networks commonly used today provide services to a plurality of devices. While a single AP can typically provide mobile communication services to multiple devices in a time-shared manner using the same channel, as discussed above each one of a number of neighboring APs uses a different channel in order to avoid interferences between APs or hidden nodes. A channel in this context corresponds to a single frequency or to a range of frequencies centered around a nominal center frequency. The physical wireless interfaces of APs and MTs are typically adapted to transmit and receive within one or more frequency ranges extending across multiple adjacent channels. The service areas of neighboring APs typically overlap so as to provide gapless communication service for a large area. The APs are connected to and communicate with each other over a backbone network, which may be wired, e.g. a local area network (LAN), or wireless. The backbone network also connects the wireless network to other networks, e.g. the Internet, providing a plurality of services to MTs attached to the wireless communication network.
IEEE802.11-enabled equipment, i.e. wireless LAN, or WLAN equipment, and also equipment adapted to communicate over other types of wireless networks, uses either active or passive discovery procedures for associating to APs. In the following, typical current implementations of the association procedure are discussed briefly. In the simplest embodiment, the MT selects the first AP it detects, i.e. when the MT is turned on, it starts to scan the channels available in the geographical area in question and selects the first AP it receives. In a more sophisticated approach, the selection is based on the information obtained from the transmissions of a plurality of APs. The MT utilizes either active or passive scanning in order to detect the APs in the region. In active scanning, the MT sends a message called a probe request on each channel. When an AP receives a probe request message, it returns a probe response to the MT. In passive scanning, the MT finds the network simply by listening for traffic on a channel, e.g. the beacon messages, which are periodically broadcast by each AP, or data traffic between other MTs and the AP. Utilizing the above-described active or passive scanning, the MT scans the channels and examines the information transmitted in the or in the other data traffic, beacon frames, or in the probe response frames, which allow for obtaining or deriving information about the properties of the AP, such as parameters indicating the security functions of the AP. The MT determines and stores a parameter termed a RSSI (Received Signal Strength Indicator) which indicates the received signal level on the link to the AP. Signal strength is an important parameter and is simple to determine even if no association is established. Also, the fact that APs apply interference monitoring and interference mitigation in their respective used channels makes signal strength an important parameter. Having finished the scanning process, the MT selects the AP with the maximum RSSI, provided that the AP fulfills other requirements set by the MT. In other words, the MT assumes that as long as the characteristics of the AP are suitable for the MT, the AP with the best RSSI provides the best quality of service.
Another important attribute of wireless networks covering a large area is the overlapping of the coverage areas, i.e. cells, of the neighboring APs, since the overlap enables roaming between the cells. When a mobile user with a MT moves beyond the coverage area of the currently serving AP, the MT must associate itself with a new AP. This process of transferring an established association from one AP to another is commonly termed re-association, or handover, and obviously also includes disassociation from a previously associated AP.
This handover process is initiated either by the AP or by the MT, whenever the AP or the MT determines that the connection, or link, is ‘bad’, i.e. no longer provides the desired service at a desired performance level, or no longer provides the service at all. For roaming, the existing association between the AP and the MT must be disassociated, either triggered by the MT or the AP, and a new association between the MT and a new AP must be established.
However, the assumption that the AP with the maximum RSSI provides the best quality of service may lead to a situation where an overwhelming majority of the MTs is associated with a few APs, while some of the APs are substantially idle. Other reasons may exist why association of MTs to one of a plurality of APs within a coverage area of a wireless network needs to be controlled by the network rather than to be left to the discretion of the algorithm implemented in the MT.
Load sharing mechanisms have been developed, which result in a more uniform load distribution between the APs, i.e. in a more even distribution of the MTs between all APs. Some existing load sharing mechanisms are based on load information sent by the APs in the beacon or probe response frames, the load information indicating the current load of the AP. The load information typically indicates the number of MTs currently associated with the AP. The load information is useful, especially in areas where the cells overlap or in congested areas requiring a multi-cell structure, i.e. where several APs cover essentially the same area.
The above-mentioned use of load information is disclosed in U.S. Pat. No. 6,469,991, for example. This document discloses a wireless communication system in which the beacon message that is broadcast from an AP includes information about the capabilities of the AP, and possibly also load metric information, which generally contains the number of MTs associated with the AP. Based on the information in the beacon message, the MT chooses the AP with which it wants to associate.
It is further known to transmit various connection attributes from the APs, the selection of the AP being based on the said attributes. International patent application WO01/63842 discloses a method in which the connection is kept in the same network as long as possible. The MT receives the said attributes from several networks and selects two APs: a first AP, which has the best connection attributes in the network that is the network of the currently serving AP and a second AP, which has the best connection attributes in another network than the network of the currently serving AP. The MT compares one or more connection attributes of the first and second APs and then re-associates with the second AP if the difference between the connection attributes of the two APs fulfills predetermined criteria. In this way, the connection can be kept in the serving network as long as possible.
Generally, a major drawback relating to the above-described known methods for joining the network and re-associating with an AP is that the decision on the correct AP can only be made on the basis of the fixed capabilities and the current load of the APs available for the MT.
Furthermore, since the present WLAN networks rely on absolute values of the attributes, such as the load of the AP or the signal level of the serving link, short-term deviations from the overall level of service in the cell may cause undesirable association or re-association decisions. For example, a short-term silent period on a channel or a short-term drop in the load level of the AP may cause such decisions.
Since the discovery procedure is typically initiated by the MT, and an AP or a network comprising a plurality of APs is merely responding to the MT-initiated discovery, another major drawback is that the final decision with which AP to associate is made in the MT, and the network has little to no control over the outcome of this process.
Yet another drawback of existing solutions is the fact that current APs are typically operational in a single channel. Thus, such AP can monitor a link quality on that channel, but cannot monitor the link quality on a different channel at the same time. Since neighboring APs typically operate on different channels, two neighboring APs cannot establish a link quality assessment with a MT within range of both APs that is associated with one of the APs.
M. E. Berezin et al., in “Multichannel Virtual Access Points for Seamless Handoffs in IEEE 802.11 Wireless Networks”, Vehicular Technology Conference (VTC Spring), 2011 IEEE 73rd, 15-18 May 2011, discloses a method of controlling handover between different APs, in which neighboring APs are switched to a currently handling APs channel and listen on that channel for determining a link quality. The listening APs communicate their results to the currently handling AP, which selects a suitable one and tells a connected MT to switch to the selected AP. This method requires accordingly adapted MTs, which can respond to a dedicated handover command.
The objective of the present invention is to alleviate or eliminate the above-mentioned drawbacks.